(1) Field of the Invention
This invention relates to an expansion valve for controlling the flow rate of refrigerant suitable for use in the refrigeration cycle of a room air conditioner, air conditioning unit in packaged form, car air conditioner and heat pump air conditioner for use as room and car air conditioning systems and packaged air conditioning systems.
(2) Description of the Prior Art
In one typical type of flow rate control valve known in the art, a plunger unitary with a valve body is attracted against the biasing force of a spring to an electromagnetic coil mounted on the valve body so as to hold the plunger in a position commensurate with the value of an electric input signal by balancing the force of attraction and the biasing force of the spring.
In this type of control valve, it is possible to control the flow rate by varying the area of a channel in the valve in accordance with the value of an electric input signal passed to the electromagnetic coil. However, magnetic hysteresis would be produced in the plunger which is formed of ferromagnetic material, so that the relation between the input voltage and the displacement of the plunger might greatly vary as the input voltage increases or decreases. Stated differently, even if the input voltage remains equal, the displacement of the plunger would show a variation, with a result that a large difference would be caused to occur in the flow rate.
Thus in this type of flow rate control valve, it is necessary to additionally provide means for correcting the aforesaid error when the valve is put to practical use. This increases cost. Also, when this valve is used with a heat pump air conditioning unit as a control valve, it functions as an expansion valve. In this case, the channel between the valve body and a valve seat is very small and high pressure differential is produced between spaces located upstream and downstream of the valve seat. In a heat pump air conditioning unit in which the direction of flow of the refrigerant is reversed in cooling and heating modes, forces exerted by the pressure differential between these two spaces on the valve body are oriented in opposite directions. Thus great difficulties would be experienced in effecting flow control in the cooling and heating modes by using a single valve, because the balance of forces between the electromagnetic coil and the spring in the cooling mode would greatly vary from that in the heating mode.
In another type of flow rate control valve which is in the form of a thermoelectric expansion valve, an electric signal is converted for one time to heat and this might cause a delay in the response of the valve body or operation. Moreover this type of valve has no ability to allow reversible flow of fluid to take place, and this makes it unfit for use with an air conditioning cycle for effecting space heating and space cooling. Furthermore, the cross-sectional area of the channel is set at a small value at the valve seat and a nozzle to throttle a flow of fluid even when the valve is in full-open position, so that resistance would be offered to the flow of the fluid through the channel.